1. Field of the Invention
The present invention relates to an optical amplifier apparatus for amplifying a signal light having wavelengths of 1.06 .mu.m band, and in particular, to a 1.06 .mu.m band optical amplifier apparatus utilizing an induced emission in an optical fiber by an excited rare-earth element.
2. Description of the Related Art
Conventionally, direct amplification of a signal light of 1.06 .mu.m band wavelengths has been implemented by using either a solid-state laser type optical amplifier (hereinafter, referred to as a first prior art example; See, for example, H. Plaessmann et al., "Multipass diode-pumped solid-state optical amplifier", Optics Letters, Vol. 18, pp. 1420-1422, 1993) or a dielectric optical waveguide type optical amplifier using quartz (hereinafter, referred to as a second prior art example; See, for example, Mitsuhiro Wada et al., "Amplification characteristic of Nd-doped integrated type optical amplifier devices by LD excitation", 1992 Spring Conference of The Institute of Electronics, Information and Communication Engineers, SB-9-1, pp. 423-424, 1992).
However, in order to obtain enough large gain with the constitution of these first and second prior art examples, it has been necessary to prolong the optical path length over which signal light and excitation light overlap each other within the crystal of an amplification medium. Since the density of the amplification medium is limited by the saturation density, it has been necessary to prolong the crystal of the amplification medium or to make the signal light reciprocate over the crystal of the amplification medium many times. As a result, the system would be increased in size as one problem. Moreover, as a further problem, it would be very hard for the second prior art example to obtain high gain more than 10 dB while the first prior art example would be more affected by shifts of the optical axis due to environmental variations. Furthermore, the gain in these prior art examples is dependent largely on polarized waves of the signal light, such that the gain would change with variations in polarized waves of the signal light as a further problem.
Further, for downsizing of the apparatus, an optical amplifier apparatus using an optical fiber doped with rear-earth elements (hereinafter, referred to as a third prior art example) has been disclosed in Japanese Patent Laid-Open Publication No. 1-94329. This apparatus of the third prior art example, however, could not able to amplify a signal light having wavelengths of 1.06 .mu.m band.